1. Field of the Invention
This invention relates implantable techniques for electrical stimulation of an organism, and more particularly, relates to such techniques for electrical stimulation of a tissue adjacent to a spinal column.
2. Description of the Related Art
Electrical leads for electrical stimulation of a spinal cord or tissue adjacent a spinal cord have been made in the past and are disclosed, for example, in U.S. Pat. Nos. 4,044,774; 5,143,067; 5,095,905; 5,031,618 and 5,121,754. Experience has shown that positional sensitivity has continued to be a problem for users of such spinal cord stimulators. Over a period of time, the stimulator may change its position with respect to the spinal column, and such positioning changes adversely affect the stimulation provided by the lead unless there is a corresponding change in the magnitude of the stimulation.
A nerve stimulator sold by Medtronic, Inc. under the name ITREL II has a "magnet amplitude" mode which allows the user to manually toggle the amplitude of the electrical stimulation between pre-programmed high and low amplitude states. U.S. Pat. No. 5,031,618 (Mullett), assigned to Medtronic, Inc., discloses a system that uses a position sensor, such as a mercury switch, to modulate the output of a neurostimulator. Neither of the foregoing systems is capable of measuring the distance between the stimulator electrodes and the spinal cord which is a key parameter affecting stimulation thresholds in the spinal cord. See, "MR Assessment of the Normal Position of the Spinal Cord in the Spinal Canal" by J. Holsheimer, et al., American Journal of Neuroradiology (May, 1994).
U.S. Pat. No. 4,706,681 (Breyer et al.) discloses marker transducers which respond to ultrasonic signals to generate electrical signals which localize the marker transducers. However, the transducers do not determine this location with respect to tissue.